When last I was able to process film (my darkroom has been out of service for about eight years), I was using my own mix "Parodinal" made from acetaminophen, sodium sulfite, and sodium hydroxide (nothing else but water), and found it would keep about six months with an inch of liquid in a pint glass jar (i.e. a lot of air relative to the volume of developer). I used to test before mixing by putting a drop of the concentrate on a film scrap in the light; if it didn't turn maximum black in about fifteen seconds, it was time to mix a new batch.
I also had some Dektol stock that I'd made up with half the usual amount of water; it was the color of Coca-Cola, but worked perfectly last time I used it, three years after mixing. I did backfill the jar with butane lighter fuel, every time I reclosed it, however.
I would expect that the "collapsing bottle", at least as encountered in chemical storage, is due to oxygen (20% of the volume of air) being taken up in oxidizing the developing agent or antioxidant -- of which, given relative densities of liquid solutions vs. air, you can do rather a lot before your developer dies. I had a bottle of Diafine Solution A that had taken that bottle collapse about as far as possible, and hadn't even visibly yellowed.
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